Epidemiological studies indicate that the amount and content of dietary lipid nutrients influence the development of colon cancer. The peroxisomal proliferator activated receptors (PPARs) are members of the retinoid-x-receptor (RXR) heterodimeric family of transcriptional regulators, and activate gene transcription in response to fatty acids and their metabolites. PPAR-gamma and PPAR-delta (beta) are in the colon and have been implicated in the pathogenesis of colon cancer. PPARs must heterodimerize with RXRs in order to bind to DNA response elements. RXRs bind 9-cis-retinoic acids, which is a vitamin A metabolite, and has recently been shown to also bind docosahexaenoic acid. RXRs also form homodimers and are obligate partners for a number of other nuclear receptors that are in intestinal epithelial cells. Thus the PPARs participate in mixed assembly of receptors that interact with RXR. We propose to test the hypothesis that RXR-PPAR heterodimers play an important role in mediating lipid nutrient signals in intestinal cells. We propose to directly measure RXR, PPAR, and RXR-PPAR heterodimer levels in intestinal cells that are grown in culture, or epithelial cells isolated from mouse colons. We plan to measure the functional consequences of altering RXR-PPAR heterodimer formation in vitro by using Caco-2 cells, which are derived from a primary human colon carcinoma and in vivo by using genetically engineered mouse models. We will search for somatic mutations and splicing variants in RXR-alpha that are associated with human colon cancers. To understand the structural basis for RXR-PPAR heterodimerization we will map the minimal peptide domain in PPAR-gamma that can disrupt RXR-PPAR interactions. We propose to study intraheterodimerization interactions between RXR and the minimal peptide by nuclear magnetic resonance techniques.